Found the source. I translate it below:
Eric Borgnis-Desbordes, Constance de Bretagne (1161-1201) : Une duchesse face à Richard Cœur de Lion et Jean sans Terre, p. 373:
"The same year [1198], Richard showed once again how secondary the question of his succession at the head of his empire was in comparison to his struggle against Philip Augustus. His nephew Otto of Brunswick (son of his sister Matilda married to Henry the Lion) had been envisaged to succeed him in the empire. Eleanor of Aquitaine especially preferred him to Arthur of Brittany. [29] Otto had the advantage of having spent most of his youth at the Angevin court and had been made Count of Poitou in 1196, precisely the year of the Breton revolt. [30] Did Richard want to show the duchess that he had a backup plan to the detriment of Arthur regarding his succession and that Constance would better submit to preserve her son’s chances?
"[29] Richard had offered Otto to marry Margaret of Scotland, daughter of William the Lion and heiress to the throne, as early as 1195 and the same year, the Archbishop of Canterbury, Chief Justiciar of the Kingdom, had proceeded to the young people’s betrothal. Eleanor had even chosen to renounce her titles in Otto’s favour, the latter appearing as Richard’s new choice, influenced by his mother. In these conditions, it is easier to understand Constance’s wariness at the idea of entrusting her son to him: in 1196, Otto calls himself Count of Poitou and Duke of Aquitaine. However, his marriage with Margaret of Scotland could not take place because of the Socttish nobility’s opposition.
"[30] This choice could also be evidence of Eleanor’s distrust towards John as Richard’s potential successor (cf. Jean-Philippe COLLET, Les vicomtes de Thouars in “Noblesses de l’espace Plantagenêt (154-1224)”, CCM, 2001, p. 156)."